In hybrid vehicles or in electric vehicles, battery packs with lithium-ion batteries are used, which consist of a large number of electrochemical battery cells connected in series. A battery management system is used to monitor the battery and should guarantee a very long lifetime besides the safety monitoring. For this purpose, it is ensured that the states of charge (State of Charge: SoC) of the individual battery cells are matched to each other despite different self-discharges. This is achieved by suitable cell symmetrization, which is referred to as cell balancing. The cell symmetrization is generally performed resistively, i.e. using at least one Ohmic resistance. For this purpose, a resistance and a switch element are associated with each battery cell in order to be able to discharge individual battery cells via said resistance, which is used for the balancing.
Besides different self-discharging rates of the individual battery cells, the capacitances of the battery cells also deviate from each other as a result of production scatter. This effect is negligibly small at the start of the lifetime of the battery cells, but can increase over the course of the lifetime as a result of differences in cell ageing and can result in capacitance differences of several percent between the individual battery cells.
In battery systems in which the capacitance of the individual battery cells is unknown and the resistive balancing is carried out to a common state of charge, the total charge to be equalized is very high, because charge is unnecessarily discharged via the balancing resistances that is far greater than for purely balancing the different self-discharges.
In the illustration according to FIG. 1, said effect is reproduced graphically. According to FIG. 1, two battery cells 16, 18 are shown with different capacitances, which according to step 1 are initially charged to 50% SoC (State of Charge). The first battery cell 16 has a lower capacitance than the second battery cell 18, which is indicated in FIG. 1 by a shorter length of dash. Charging takes place in step 2. In step 3 balancing back to the same SoC (State of Charge) takes place, i.e. the first battery cell is resistively partially discharged. In the following step 4 discharging of the battery cells 16, 18 takes place. Then the second battery cell 18 must be resistively partially discharged in order to achieve a consistent SoC (State of Charge) relative to the first battery cell 16. The need for balancing during the transition from step 4 to step 5 is partly created by the balancing during the transition from step 2 to step 3. If the battery system is continuously balanced such that all battery cells have an identical equal SoC, not only is charge unnecessarily discharged via the balance resistances required for equalization, i.e. for balancing, but rather there are also unnecessarily many switching processes of the balancing unit, which can adversely affect its service life.
DE 10 2009 045 519 A1 relates to a battery system and a method for balancing the battery cells of said battery system. The battery system comprises a first battery element. The positive pole of the first battery element is conductively connected to the negative pole of the second battery element. A discharging means is provided for the partial discharging of the first and second battery elements. A potential divider is designed, starting from the electrical potential of the negative pole of the first battery element and the electrical potential of the positive pole of the second battery element, to generate a first electrical potential that corresponds to the target value of the electrical potentials on the positive pole of the first battery element and the negative pole of the second battery element. A comparison means is used to compare the first electrical potential with a second electrical potential that is applied to the positive pole of the first battery element and the negative pole of the second battery element. The discharging means is designed to discharge the first battery element if the second electrical potential deviates from the first electrical potential in the positive direction and to discharge the second battery element if the second electrical potential deviates from the first electrical potential in the negative direction.